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If you have an older truck with no overflow bottle, there should at least be a small outlet tube under the radiator cap that is connected to a tube that directs the overflow to the ground. If your truck is like mine when I got it, there was no hose/tube. The fluid just puked all over the top of the radiator. Look for crusty residue the color of your coolant around that outlet or all over the radiator fins. Or add a bottle so you can keep an eye on it.
Somebody more well-versed than me can confirm this, but around 1975(?), Toyota switched over to an overflow bottle and a different cap that was designed to let the coolant overflow at high temps, then draw it back into the radiator as it cooled off. It's my understanding that if you want to add a functional overflow bottle that allows this two-way flow of fluid, you need to replace the cap with the later cap designed for this function.